The present invention relates to a passive polarisation modulating optical element and to an optical device including such an element. The present invention also relates to a method of making a passive polarisation modulating optical element. Such an element may be used in three dimensional (3D) displays, for instance of the autostereoscopic type. Such displays may be used in games apparatuses, computer monitors, laptop displays, work stations and professional imaging, for instance for medical, design or architectural use.
The present invention relates to a parallax barrier and to a display. Such displays may be used as switchable two dimensional (2D)/three dimensional (3D) displays and may be used in games apparatuses, computer monitor, lap top displays, work stations and professional imaging, for instance for medical, design or architectural use.
In normal vision, the two human eyes perceive views of the world from two different perspectives due to their spatial separation within the head. These two perspectives are then used by the brain to assess the distance to various objects in a scene. In order to provide a display which effectively displays a 3D image, it is necessary to recreate this situation and supply a so-called xe2x80x9cstereoscopic pairxe2x80x9d of images, one to each eye of an observer.
Most 3D displays may be classified into two types depending on the technique used to supply the different views to the eyes. Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is eIicuded, fur instance by colour, polarisation state or time of display, so that a filter system of glasse worn by the observer attempts to separate the views to let each eye see only the view that is intended for it.
Autostereoscopic displays require no viewing aids to be worn by the observer. Instead, the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a xe2x80x9cviewing regionxe2x80x9d. If the observer is situated such that one eye is in one viewing region and the other eye is in the other viewing region, then a correct set of views is seen and a 3D image is perceived.
For autostereoscopic displays of the xe2x80x9cflat panelxe2x80x9d type, the viewing regions are formed by a combination of the picture element (pixel) structure of the display and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier. This element is a screen with vertical transmissive slits separated by opaque regions. A display of this type is illustrated in FIG. 1 of the accompanying drawings. A spatial light modulator (SLM) 1 of the liquid crystal type comprises glass substates 2 between which are disposed a liquid crystal layer together with associated electrodes and alignment layers. A backlight 3 illuminates the SLM 1 from behind and a parallax barrier 4 is disposed on the front surface of the SLM 1.
The SLM 1 comprises a 2D array of pixel apertures with the pixels arranged as columns as shown at 5 separated by gaps 6. The parallax barrier 4 has vertically extending slits 7 with a horizontal pitch close to an integer multiple of the horizontal pitch of the pixel columns 5 so that groups of columns of pixels are associated with each slit. As illustrated in FIG. 1, three pixel columns labelled columns 1, 2 and 3 are associated with each slit 7 of the parallax barrier 4.
The function of the parallax optic such as the parallax barrier 4 is to restrict the light transmitted through the pixels to certain output angles. This restriction defines the angle of view of each of the pixel columns behind the associated slit. The angular range of view of each pixel is determined by the pixel width and the separation between planes containing the pixels and the parallax optic. As shown in FIG. 1, the three columns 5 associated with each slit 7 are visible in respective viewing windows.
FIG. 2 of the accompanying drawings illustrates the angular zones of light created from an SLM 1 and a parallax barrier 4 where the parallax barrier slits have a horizontal pitch equal to an exact integer multiple of the pixel column pitch. In this case, the angular zones coming from different locations across the display surface intermix and a pure zone of view for image 1 or image 2 does not exist. Thus, each eye of an observer will not see a single image across the whole of the display but instead will see slices of different images at different regions on the display surface. In order to overcome this problem, the pitch of the parallax optic is reduced slightly so that the angular zones converge at a predetermined plane, generally known as the xe2x80x9cwindow planexe2x80x9d, in front of the display. This change in the parallax optic pitch is termed xe2x80x9cviewpoint correctionxe2x80x9d and is illustrated in FIG. 3 of the accompanying drawings. The window plane is shown at 8 and the resulting substantially kite shaped viewing regions are shown at 9 and 10. Provided the left and right eyes of the observer remain in the viewing regions 9 and 10, respectively, each eye will see the single image intended for it across the whole of the display so that the observer will perceive the 3D effect,
The window plane 8 defines the optimum viewing distance of the display. An observer whose eyes are located in this plane receives the best performance of the display. As the eyes move laterally in this plane, the image on the display remains until the eyes reach the edge of the viewing regions 9 and 10, whereupon the whole display swiftly changes to the next image as one eye moves into the adjacent viewing region. The line of the window plane within each viewing region Is generally termed a xe2x80x9cviewing windowxe2x80x9d.
FIG. 4 of the accompanying drawings illustrates an autostereoscopic display which differs from that shown in FIG. 1 in that the parallax barrier 4 is disposed on the rear surface of the SLM 1. This arrangement has the advantage that the barrier 4 is disposed behind the SLM 1 away from possible damage. Also, the light efficiency of the display may be improved by making the opaque parts of the rear surface of the parallax barrier 4 reflective so as to recycle light which is not incident on the slits 7.
A switchable diffuser 11 is shown between the parallax barrier 4 and the SLM 1. Such a diffuser may comprise a polymerispersed liquid crystal which is switchable between a low scattering or substantially clear state and a highly scattering state. In the low scattering state, the display operates as described hereinbefore as an autostereoscopic 3D display. When the diffuser is switched to the highly scattering state, light rays are deflected on passing through the diffuser and form an even or xe2x80x9cLambertianxe2x80x9d distribution which xe2x80x9cwashes outxe2x80x9d the effect of the parallax barrier 4 and so destroys the creation of viewing regions. In this mode, the display therefore acts as a conventional 2D display with the full spatial resolution of the SLM 1 being available for displaying 2D images.
In the displays described hereinbefore, the basic principle is that a subset of the total number of pixels of the SLM 1 is visible to each eye at any one time. Thus, each of the views represented in the viewing regions uses a fraction of the total resolution of the SLM 1. In a typical two view spatially multiplexed autostereoscopic display, each eye perceives an image of only half the total resolution. For a three view system, the resolution in each eye is only one third. The representation of complex small characters, such as text and details within images, may therefore be adversely affected. It is desirable to include in the display some means for disabling or overcoming the parallax imaging system so that the full resolution of the SLM 1 is visible to each eye for the display of detailed 2D information. Although the switchable diffuser 11 shown in FIG. 4 provides such switching, this adds to the cost and complexity of the display.
U.S. Pat. No. 2,631,496 discloses an autostereoscopic display based on a single picture in which a parallax element is provided by a polariser element having alternate stripes of orthogonally oriented polariser. The polariser element co-operates with an image in which the left and right views are encoded with orthogonal polarisations in vertical columns. The encoding swaps for every image strip column. The polariser element thus acts in a similar manner to a parallax barrier but is such that the mark/space ratio i.e. the ratio of the width of each effective slit to each effective opaque region, is substantially equal to 1. This results in relatively high cross talk and poor viewing freedom for the observer. Such an arrangement does not permit a full resolution 2D viewing mode to be achieved without image artefacts.
Proc. SPIE vol. 2177, pp 181 xe2x80x9cNovel 3D Stereoscopic Imaging Technologyxe2x80x9d, S. M. Faris, 1994 discloses a display which may operated stereoscopically or autostereoscopically using external micropolarisers. In particular, two micropolariser sheets are disposed above the spatially multiplexed image and are movable to switch between autostereoscopic and stereoscopic viewing. Such an arrangement cannot be operated to provide a high resolution 2D viewing mode.
E. Nakayama et al, xe2x80x9c2D/3D Compatible LC Display without Special Glassesxe2x80x9d, Proc. third Internal Display Workshops vol. 2, pp 453-456, 1996 discloses a 3D display of the rear parallax barrier type similar to that shown in FIG. 4 of the accompanying drawings. A switchable diffuser is disposed between the parallax barrier and the SLM in the same way as illustrated in FIG. 4 to allow the display to be operated in a full resolution 2D mode.
In order to destroy the formation of viewing windows for the 2D mode, scattering by the diffuser must completely remove the visibility of the parallax barrier to the observer. However, in order for the autostereoscopic 3D mode to be effective, the gaps between the slits of the parallax barrier must provide strong extinction of light. These requirements are mutually incompatible and can be overcome only by very strong back-scattering in the switchable diffuser, which reduces the display transmission substantially, or by making the parallax barrier reflective on the observer side, thus damaging the 3D image. Further, although a rear reflective layer may be applied to the parallax barrier so as to recycle light and improve brightness, all of the light received by the observer has to pass through the slits of the parallax barrier so that display brightness is degraded in the 2D mode. Typically, the mark space ratio of the parallax barrier would be 2:1 so that only one third of the light from the backlight is transmitted through the display. The reflective layer may improve this but would not restore the display to full brightness. Further, back scatter in the switchable diffuser would reduce the display brightness in the 2D mode. If the switchable diffuser is designed for strong backscatter in the high diffusion mode of operation, it is difficult to achieve the very low levels of diffusion necessary in the low diffusion mode to ensure that the 3D display device does not suffer from increased cross talk. J. B. Eichenlaub, Proc. SPIE 2177, pp 4-15, xe2x80x9cAn Autostereoscopic Display with High Brightness and Power Efficiencyxe2x80x9d, 1994 discloses a 3D display of the rear parallax barrier type which could be switched to a full resolution 2D mode using a switchable diffuser or an array of lamps. However, such an arrangement has the disadvantages described hereinbefore. Furthermore, the optical system of such a display is not compatible with the slim design of current flat-panel display systems wherein the backlight structure is less than 1 cm thick.
U.S. Pat. No. 5,264,964 discloses a passive display of the rear parallax barrier type. The display is switchable between stereoscopic and autostereoscopic modes of viewing. The rear parallax barrier comprises two micropolarisers with a nematic liquid crystal layer there between. The micropolarisers have aligned polarising and non-polarising regions such that, when the liquid crystal is in its inactive state and has not effect on the polarisation of light, polarising glasses have to be worn in order to view the image stereoscopically. When the liquid crystal layer is in its active state, it rotates the polarisation of light through 90xc2x0. The aligned polarising regions of the micropolarisers then block light so that a rear parallax barrier is formed and the image can be viewed autostereoscopically.
When the display is in the 2D mode, light enters the liquid crystal layer from both polarised and unpolarised regions of the input micropolariser. The polarised regions have a lower transmissivity than the unpolarised regions and this causes Moire effects in the illumination of the display. This results in illumination stripes in the display and flickering illumination as the observer moves. The 2D image appearance will therefore be very poor. Further, image pixels associated with one polarisation direction do not transmit light from barrier regions of the orthogonal polarising direction. This causes further illumination non-uniformities and causes obstruction of vertical pixel lines.
In the 3D mode of this device, opaque regions are coloured and transmit a significant quantity of light because of problems of the polarisation change varying with the wavelength of the incident light. The polarisation is rotated by 90xc2x0 at only one xe2x80x9cdesignxe2x80x9d wavelength. At other wavelengths, the rotation is approximate. This results in significant cross talk levels which give poor 3D image quality. Also, the mark/space ratio of the barrier is 1:1 which results in limited viewing freedom and high levels of cross talk.
xe2x80x9cMolecular architectures in thin plastic film by in-situ photopolymerisation or reactive liquid crystalsxe2x80x9d Phillps SID 95 Digest discloses a method of making patterned optical waveplates.
xe2x80x9cSurface induced parallel alignment of liquid crystals by linearly polymerized photopolymersxe2x80x9d Schadt et al Japanese Journal of Applied Physics, vol 31, 1992, pp 2155 discloses a technique based on the photopolymerisation of liquid crystals obtained by crosslinking polyvinylmethoxycinnamate using polarised light.
EP 0 689 084 discloses the use of reactive mesogen layers as optical elements and alignment surfaces.
U.S. Pat. Nos. 5,537,144 and 5,327,285 disclose photolithographic techniques of patterning polarisers or retarders. An array of waveplates is generated by bleaching a stretched film of PVA through a photoresist mask in a hot humid atmosphere or with water-based bleachers. This alters the material properties so that the retardance properties of the material are selectively destroyed in certain regions. Thus, such a technique may be used to provide a single layer element in which some regions act as retarders with the optic axes parallel to each other and other regions have substantially zero retardance.
xe2x80x9cFour domain TNCLD fabricated by reverse rubbing or double evaporationxe2x80x9d Chen et al SID 95 Digest page 865 discloses the use of a technique involving double-rubbing of an alignment layer in an active liquid crystal device (LCD). The liquid crystal alignment direction varies within each pixel to enable improved viewing angle performance of the device.
According to a first aspect of the invention, there is provided a parallax barrier characterised by comprising: a polarisation modifying layer having aperture regions, for supplying light of a second polarisation when receiving light of a first polarisation, separated by barrier regions, for supplying light of a third polarisation different from the second polarisation when receiving light of the first polarisation, at least one of the aperture regions and the barrier regions altering the polarisation of light passing therethrough; and a polariser selectively operable in a first mode to pass light of the second polarisation and to block light of the third polarisation and in a second mode to pass light of the third polarisations.
Such a parallax barrier can therefore be operated in a parallax barrier mode or in a non-barrier mode. When illuminated by light of the first polarisation, the non-barrier mode permits substantially all of the light to be transmitted so that, when used in a 3D autostereoscopic display, a full resolution 2D mode of high brightness can be provided.
The aperture regions may comprise parallel elongate slit regions.
The polariser may be a uniform polariser.
The third polarisation may be orthogonal to the second polarisation.
The first, second and third polarisations may be linear polarisations. The aperture regions may be arranged to rotate the polarisation of light and the barrier regions may be arranged not to rotate the polarisation of light so that the third polarisation is the same as the first polarisation. Such an arrangement allows the barrier regions to have maximum achromatic extinction of light when the barrier is used in barrier mode.
The aperture regions may comprise retarders. The aperture regions may comprise half waveplates. As an alternative, the aperture regions may comprise polarisation rotation guides.
The polarisation modifying layer may comprise a half waveplate, the aperture regions may have optic axes aligned at xc2x1 substantially 45xc2x0 to the first polarisation, and the barrier regions may have optic axes aligned substantially parallel to the first polarisation.
The polariser may pass light of the second polarisation in the second mode.
The polariser may be removable from a light path through the polarisation modifying layer in the second mode. The polariser does not have to be aligned with great accuracy in order for the barrier mode to be effective. In particular, it is merely necessary for the polariser to cover the polarisation modifying layer and to be reasonably accurately aligned rotationally about an axis substantially normal to the layer. Thus, removal of the polariser permits the non-barrier mode of operation and relatively simple and inexpensive alignment means may be provided for aligning the polariser in the barrier mode.
The polariser may comprise glasses to be worn by an observer in the first mode.
The polariser may be rotatable through substantially xc2x0 an axis substantially perpendicular to the polarisation modifying layer between first and second positions for operation in the first and second modes, respectively.
The polariser may comprise a polarising layer and a retarder layer which is switchable between a non-retarding mode and a retarding mode providing a quarter wave of retardation.
The polariser may comprise a polarising layer and a switchable diffuser having a diffusing depolarising mode and a nondiffusing non-depolarising mode. The diffuser may be disposed between the polarising layer and the polarisation modifying layer. As an alternative, the polarisation modifying layer may be disposed between the polarising layer and the diffuser.
The barrier may comprise: a first quarter waveplate disposed between the polarisation modifying layer and the polariser and attached to the polarisation modifying layer; and a second quarter waveplate disposed between the first quarter waveplate and the polariser and attached to the polariser, the first and second quarter waveplates having substantially orthogonal optical axes. The quarter waveplates between the polarisation modifying layer and the polariser convert light to and from circular polarisation so that rotational alignment of the polariser relative to the polarisation modifying layer may be further relaxed.
According to a second aspect of the invention, there is provided a display comprising a barrier according to the first aspect of the invention and a spatial light modulator for supplying light of the first polarisation to the polarisation modifying layer.
The spatial light modulator may be a light emissive device, such as an electroluminescent display. As an alternative, the spatial light modulator may provide selective attenuation of light and may be associated with a light source. The spatial light modulator may comprise a liquid crystal device.
According to a third aspect of the invention, there is provided a display comprising a barrier according to the first aspect of the invention, a light source for supplying light to the polariser, and a spatial light modulator having an input polariser for passing light from the aperture regions.
The spatial light modulator may comprise a liquid crystal device.
According to a fourth aspect of the invention, there is provided a display comprising: a light source selectively operable in a first mode for supplying light of a first polarisation and a second mode for supplying unpolarised light; a polarisation modifying layer having aperture regions, for supplying light of a second polarisation when receiving light of the first polarisation, separated by barrier regions, for upplying light of a third polarisation different from the second polarisation when receiving light of the first polarisation; and a spatial light modulator having an input polariser for passing light of the second polarisation and for blocking light of the third polarisation.
The aperture regions may comprise parallel elongate slit regions.
The light source may comprise a polarised light source operable in the first mode and an unpolarised light source operable in the second mode. The polarised light source may comprise at least one first light emitting device arranged to supply light through a polariser to a first light guide. The unpolarised light source may comprise at least one second light emitting device arranged to supply light to a second light guide and one of the first and second light guides may be arranged to supply light through the other of the first and second light guides.
The light source may comprise at least one light emitting device, a light guide, and a polariser disposed in an optical path between the or each light emitting device and the light guide in the first mode and outside the optical path in the second mode.
According to a fifth aspect of the invention, there is provided a display comprising: a polarisation modifying layer having aperture regions, for supplying light of a second polarisation when receiving light of a first polarisation, separated by barrier regions, for supplying light of a third polarisation different from the second polarisation when receiving light of the first polarication; a spatial light moculator having an input polariser for passing light of the second polarisation and for blocking light of the third polarisation; a light source; a mask having polarising regions, for supplying light of the first polarisation from the light source, and non-polarising regions, for transmitting light from the light source; and a parallax optic co-operating with the mask to direct light from the polarising regions through the spatial light modulator to a first viewing region and to direct light from the non-polarising regions through the spatial light modulator to a second viewing region.
The mask may be movable relative to the parallax optic for moving the first and second viewing regions.
The parallax optic may comprise an array of parallax generating elements.
The aperture regions may comprise parallel elongate slit regions.
Each of the parallax generating elements may be optically cylindrical with an axis substantially orthogonal to the slit regions.
The array may comprise a lenticular screen. As an alternative, the array may comprise a parallax barrier.
The polarising and non-polarising regions may comprise laterally extending strips.
The mask may further comprise opaque regions at least partially separating the polarising regions from the non-polarising regions.
The third polarisation may be orthogonal to the second polarisation.
The first, second and third polarisations may be linear polarisations. The aperture regions may be arranged to rotate the polarisation of light and the barrier regions may be arranged not to rotate the polarisation of light so that the third polarisation is the same as the first polarisation.
The aperture regions may comprise retarders.
The aperture regions may comprise half waveplates.
The aperture regions may comprise polarisation rotation guides.
It is thus possible to provide a display, for instance of the flat panel type which is operable in a wide view full resolution 2D mode and in a directional 3D autostereoscopic mode. When embodied as a liquid crystal device whose pixel apertures are at least partially defined by a black mask, there are no undesirable visual artefacts associated with the black mask in the 2D mode.
The pitch alignment of the polarisation modifying layer determines the parallax barrier pitch, which typically has to be set to within 0.1 micrometers. The barrier may be made of a glass substrate with similar thermal expansivity to the spatial light modulator so as to minimise misalignments during heating of the system between switch on and operating temperatures. The high tolerance alignment can be fixed during manufacture and is unaffected by switching between 2D and 3D modes. There are six critical degrees of freedom alignment tolerances in such displays with respect to the positioning of the apertures of the barrier relative to the spatial light modulator and these do not have to be set in the field. Because the removable or switchable element can be a uniform polarisation element, accurate alignment is only necessary in one degree of freedom i.e. rotation about an axis normal to the display surface. Rotation about the other two axes and spatial positioning may all be set with low and easy to satisfy tolerance requirements. Thus mechanical assembly is substantially simplified and cost, size and weight can be reduced.
It is possible to switch different regions of the display independently to allow 3D and 2D regions to be mixed simultaneously on the display surface.
A colour 3D display can be provided with low cross talk using relatively simple and inexpensive birefringent elements. The 2D mode may be substantially as bright as a conventional display with the same angle of view. Thus, the same backlight as for a conventional display may be used and battery life and brightness will not be compromised. An anti-reflection coating may be applied to the outside surface to reduce reflections and improve display contrast. There are minimal absorption or reflection losses from such an additional layer.
When applied to an observer tracking display, the tracking may be performed by relative movement between the spatial light modulator and the polarisation modifying layer. Thus, the polarisation modifying layer may remain attached to the mechanical system at all times. The polariser does not need to be attached to the mechanical system at all so that mounting is simplified. In fact, the polariser does not need to be mounted in physical proximity to the polarisation modifying layer and may indeed be provided in the form of glasses to be worn by an observer.
According to a first aspect of the invention, there is provided a passive polarisation modulating optical element comprising a layer of birefringent material having substantially fixed birefingence and comprising at least one first retarder having an optic axis aligned in a first direction and at least one second retarder having an optic axis aligned in a second direction different from the first direction.
The at least one first retarder may comprise a plurality of first retarders, the at least one second retarder may comprise a plurality of second retarders, and the first and second retarders may be arranged as a regular array. The first and second retarders may comprise first and second strips which alternate with each other, The first strips may have a first width and the second strips may have a second width less than the first width.
The first and second retarders may have a retardance of (m+1)xcex/2, where m is an integer and xcex is a wavelength of visible light.
The second direction may be at substantially 45xc2x0 to the first direction.
The birefringent layer may be disposed of an alignment layer comprising first and second regions corresponding to the first and second retarders, respectively, and hating first and second alignment directions, respectively.
The birefringent material may comprise a reactive mesogen.
According to a second aspect of the invention, there is provided an optical device comprising an element according to the first aspect of the invention and a linear polariser for passing light polarised at a predetermined angle with respect to the first optic axis.
The predetermined angle may be substantially equal to 0xc2x0.
The polariser may comprise part of a further device. The further device may be a liquid crystal device.
According to a third aspect of the invention, there is provided a method of making a passive polarisation modulating optical element, comprising forming an alignment layer, providing at least one first region of the alignment layer with a first alignment direction, providing at lease one second region of the alignment layer with a second alignment direction different from the first alignment direction, disposing on the alignment layer a layer of birefringent material whose optic axis is aligned by the alignment layer, and giving the optic axis of the birefringent layer.
The at least one first region may comprise a plurality of first regions, the at least one second region may comprise a plurality of second regions, and the first and second regions may be arranged as a regular array. The first and second retarders may comprise first and second strips which alternate with each other. The first ships may have a first width and the second strips may have a second width less than the first width.
The birefringent layer may have a thickness for providing a retardance of (m+1)xcex/2, where m is an integer and xcex is a wavelength of visible light./
The second direction may be substantially at 45xc2x0 to the first direction.
The birefringent material may comprise a reactive mesogen.
The fixing may be performed by irradiation. The fixing may be performed by ultraviolet irradiation.
The alignment layer may comprise polyimide.
The whole of the alignment layer may be provided with the first alignment direction, after which the or each second region may be altered to have the second alignment direction. The alignment layer may be rubbed in a first rubbing direction, the alignment layer may be masked to reveal the or each second region, and the or each second region may be rubbed in a second rubbing direction.
The alignment layer may comprise a linearly photopolymerisable polymer, the alignment layer may be masked to reveal the or each first region, the or each first region may be exposed to radiation having a first linear polarisation, the alignment layer may be masked to reveal the or each second region, and the or each second region may be exposed to radiation having a second linear polarisation different from the first linear polarisation.
Such an optical element may be used, for instance, to provide a parallax barrier which may be used in an autogtereoicopic display and whose parallax barrier operation may be disabled to permit such a display to be used in a two dimensional (2D) mode. A device of this type is disclosed in British patent application No: 9713985.1. When in the 2D mode, it is advantageous to avoid any difference in light absorption between the regions which act as the slits in the 3D mode and the regions between the slits. Otherwise, in the 2D modes visible Moire patterning could be produced by beating of the variation in absorption with the pixel structure or the display.
The optical element may be made using a single photolithographic mask step, thus reducing the complexity of manufacture and the cost of the element. The element may be bonded to another substrate so as to avoid damage to its surface without affecting the optical properties of the element. The element may be formed on a glass substrate which allows the application of a low-cost anti-reflection layer on the opposite surface substrate prior to making the element.
The optical element may be manufactured using existing processes, such as spin coating, photolithographic masking and rubbing techniques. Thus, optical elements of this type may be manufactured in high volume at low cost. The element is manufactured without the removal of retarder material and so can be more easily made without introducing surface artefacts or damage and without requiring subsequent planarisation. Bu using photolithographic techniques, the retarder regions may be formed with high accuracy and resolution so that such an element is suitable for use in a viewpoint corrected parallax barrier. Further, it is possible to provided an element having high levels of dimensional stability.